A typical sol-gel process involves the transition of a liquid colloidal solution “sol” phase into a solid porous “gel” phase, followed by drying and sintering the resulting gel monolith at elevated temperatures. The conventional sol-gel process for the formation of ceramic or glass materials consists of hydrolysis of a metal alkoxide precursor, typically tetramethylorthosilicate or tetraethylorthosilicate for forming silica, in the presence of an acid or base catalyst. The reaction mixture is placed in a desired receptacle and undergoes hydrolysis and polymerization, resulting in a wet porous gel monolith or matrix formed in situ. After drying the wet gel monolith in a controlled environment to remove the fluid from the pores, the dry gel monolith can be calcined into a solid glass-phase monolith. The materials produced by this process display connected open pores with a generally narrow range of the pore size distribution. This type of air-dried xerogel typically possesses numerous pores or channels well below 15 Å in diameter, depending on the synthesis conditions.
In the use of these porous materials as support or separation devices, the average size and size distribution of pores should be precisely controlled so as to optimize the function of supported substances or the separation efficiency. For many applications the porous material should contain a defined mesopore size distribution in addition to the network of macropores present. To that end, many investigators have attempted to control the size and distribution of macropores by adjusting the reaction parameters of gel preparation, including adding pore forming agents during gel preparation, or remodeling the interior surface to enlarge and/or provide a more uniform size distribution of mesopores.
For example, the influence of the catalyst concentration on the pore sizes of the resultant gel monoliths is illustrated by U.S. Pat. No. 5,264,197 to Wang, which describes preparing a sol gel monolithic material by adjusting the relative concentrations of an alcohol and/or catalysts such as HCl or HF at concentrations up to a maximum of about 0.05 moles per mole of tetraethoxysilane. This patent describes that when the alcohol is ethanol and the catalysts are hydrofluoric acid and hydrochloric acid, the average pore radii in the dry gel can be tailored to selected values in the range of 10 Å to 100 Å by controlling the relative concentrations of the ethanol and the catalyst. Correspondingly, the gel surface area is reported to be tailored to values in the range of 600 to 1100 m2/g. However, the process does not produce enough macropores which are generally required for chromatographic separations.
U.S. Patent Application Publication Nos. 2003/0068266 and 2003/0069122 describe that the use of HF can promote the formation of larger pore sizes, thus reducing the tendency for cracking of gel monoliths. However, the inventors point out that the use of catalysts such as HF also shortens gelation times, and can result in insufficient time for processing, or for bubbles to diffuse out of the gel, thereby degrading the quality of the gel produced. A method of manufacturing a xerogel monolith is described that includes preparing a first solution comprising metal alkoxide, a second solution comprising a catalyst, and mixing the first and second solutions together, where at least one of the solutions is cooled to achieve a mixture temperature for the third solution which is substantially below room temperature. In so doing, the mixture reportedly has a significantly longer gelation time at the mixture temperature as compared to a room temperature.
Another approach is described in U.S. Pat. No. 5,624,875 to Nakanishi, which describes the solidification of the solution to form a sol gel, the aging of the gel for an appropriate period, and then the immersion of the gel in a matrix dissolving agent, such as sodium hydroxide, aqueous ammonia, or hydrofluoric acid. This patent states that during the immersion process, substitution of external solution with the solvent-rich phase takes place, allowing contact of the external solution with the inner-surface of silica-rich phase, and that when the external solution can dissolve the matrix, the inner wall is subjected to a dissolution and re-precipitation process, resulting in the loss of smaller pores and the increase of larger pores. This patent states that this step is essential for creating sharply distributed mesopores. Thus, this patent demonstrates that the sol gel must first be formed and then immersed in a matrix dissolving agent in order to obtain the desired mesopore distribution, a time consuming and difficult to control step.
Similarly, U.S. Pat. No. 6,207,098 to Nakanishi reports a process for producing inorganic porous materials composed of glass or glass ceramic components reportedly having interconnected continuous macropores with a median diameter larger than 0.1 μm and mesopores in the walls of said macropores having a median diameter between 2 and 100 nm. The process reportedly includes (a) dissolving a water-soluble polymer or other pore forming agent and a precursor for a matrix dissolving agent in a medium that promotes the hydrolysis of an organometallic compound; (b) mixing with an organometallic compound which contains hydrolyzable ligands; (c) solidifying the mixture through the sol-gel transition, whereby a gel is prepared which has three dimensional interconnected phase domains one rich in solvent the other rich in inorganic component in which surface pores are contained; (d) setting the matrix dissolving agent free from its precursor, whereby the matrix dissolving agent modifies the structure of said inorganic component; (e) removing the solution by evaporation drying and/or heat-treatment; (f) calcining the gel to form the porous material. However, it is very difficult to eliminate micropores using this process, limiting the performance during chromatographic separations. Further, this process requires preparing a gel and then performing an additional step to modify the structure of the gel, which is a complicated and time consuming procedure.
U.S. Pat. Nos. 6,562,744 and 6,531,060 to Nakanishi further describe inorganic porous materials contained in a confined space having at least one dimension less than 1 mm across and in liquid tight contact with the walls of the container, such as a capillary. The process involves thermally decomposing a component that modifies the gel structure, such as an amide compound that is capable of making the reaction system basic when the compound is thermolysed.
U.S. Pat. No. 6,398,962 to Cabrera further describes using the method of Nakanishi for preparation of a monolithic sorbent for use in simulated moving bed chromatography. The monolithic sorbent is reportedly based on shaped SiO2 bodies having macropores of diameter from 2 to 20 μm and mesopores of diameter from 2 to 100 nm. However, as described above, the process for preparing the monoliths is laborious and time consuming.
U.S. Patent Application Publication No. 2003/0150811 describes a porous inorganic/organic hybrid material and a process for forming the same wherein the pores of diameter less than about 34 A reportedly contribute less than 110 m2/g to less than 50 m2/g to the specific surface area of the material. The process reportedly involves forming porous inorganic/organic hybrid particles, modifying the pore structure of the porous hybrid particles, and coalescing the porous hybrid particles to form a monolith material. This application also reports the hydrothermal treatment of hybrid monolithic silica, formed in a similar process as described in the above patents, in order to modify the pore structure. However, these processes are laborious and time consuming, and may not eliminate micropores.
U.S. Patent Application Publication No. 2001/0033931 assigned to Waters describes porous inorganic/organic hybrid particles reportedly having a chromatographically-enhancing pore geometry. The process for preparing the porous particles reportedly involves the three step process of forming the particles, suspending the particles in an aqueous medium in surfactant and gelling the particles into porous spherical particles of hybrid silica using a base catalyst, and modifying the pore structure by hydrothermal treatment. The process is thus laborious and time consuming.
Sol gel monoliths are also subject to cracking and shrinking during the drying step of the fabrication process. Approaches to reduce the cracking have been attempted, focused on increasing the pore sizes of the gel monolith to reduce the capillary stresses generated during drying. For example, U.S. Pat. No. 5,023,208 to Pope describes subjecting the gel to a hydrothermal aging treatment, which reportedly causes silica particles to migrate and fill small pores in the porous gel matrix, and increase the average pore size. U.S. Pat. No. 6,210,570 to Holloway describes that “syneresis,” or the shrinkage in volume as a hydrosol progresses to a hydrogel, can occur to the extent that a volume of a material can decrease by a factor of 100. U.S. Pat. No. 6,620,368 to Wang describes that the density of the gel at the end of the first stage of liquid removal process corresponds to a shrinkage in the linear dimension of between about 15% and 35%.
U.S. Pat. No. 6,528,167 to O'Gara describes a method of preparing chromatographic particles for performing separations or for participating in chemical reactions, including: (a) prepolymerizing a mixture of an organoalkoxysilane and a tetraalkoxysilane in the presence of an acid catalyst to produce a polyalkoxysiloxane; (b) preparing an aqueous suspension of the polyalkoxy siloxane further comprising a surfactant, and gelling in the presence of a base catalyst so as to produce porous particles having silicon C1-7 alkyl groups, substituted or unsubstituted aryl groups, substituted or unsubstituted C1-7, alkylene, alkenylene, alkynylene, or arylene groups; (c) modifying the pore structure of the porous particles by hydrothermal treatment; and (d) replacing one or more surface C1-7 alkyl groups, substituted or unsubstituted aryl groups, substituted or unsubstituted C1-7 alkylene, alkenylene, alkynylene, or arylene groups of the particle with hydroxyl, fluorine, alkoxy, aryloxy, or substituted siloxane groups. The replacing step involves reacting the hybrid particle with aqueous H2O2, KF, and KHCO3 in an organic solution, which may further include a porogen.
U.S. Pat. No. 6,346,140 to Miyazawa describes a process for preparing a porous solid for gas adsorption separations wherein the micropore volume is at least 10% and preferably from 20% to 50% of the total pore volume. This patent also describes porous solids having total micropore volumes of at least 0.05 cc/g, and mesopore volumes of 0.25 to 0.58 cc/g. In addition, this patent teaches the importance of limiting the surfactant below a concentration of 29 g/l in order to allow formation of micropores. Thus this patent teaches the production of a sol gel having significant micropores.
Thus, numerous processes for preparing sol gel monolithic sorbents are known in the art of chromatographic separations. However, production of sorbents having the desired distribution of macro- and mesopores with substantially no micropores remains an unsolved problem. In addition, the procedures known in the art are complicated and difficult to control, costly and time consuming. Therefore, there is a need in the art for methods of producing ultraporous sol gel monolithic sorbents providing superior flow characteristics and having the desired distribution of macro- and mesopores with substantially no micropores. In addition, there is a need in the art for procedures that are simple and uncomplicated, provide good control over the reaction and the products, and that are less costly and time consuming to produce and to use.